<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .btn{
            display: inline-block;
            padding: 5px 10px;
            background-color: coral;
            color: white;
        }
    </style>
</head>
<body>

    <iframe style="display: none" id="iframe1" name="ifra1"></iframe>
    <form id="fm1" action="/fs/upload_img.html" method="POST" enctype="multipart/form-data" target="ifra1">
        <input type="file" name="k3" onchange="uploadFile();" />
    </form>
    <h3>预览</h3>
    <div id="preview">
    </div>
    <script src="/static/jquery-3.3.1.js"></script>
    <script>

        function uploadFile() {
            document.getElementById('iframe1').onload = reloadIframe1;
            document.getElementById('fm1').submit();
        }
        function reloadIframe1() {
            var content = this.contentWindow.document.body.innerHTML;
            var obj = JSON.parse(content);

            var tag = document.createElement('img');
            tag.src = obj.data;
            $('#preview').empty().append(tag);
        }
    </script>
</body>
</html>